Structural organization and cytochemical features of the pistil in Olive (Olea europaea L.) cv. Picual at anthesis

Abstract

Pistil structure and composition are critical in recognizing and permitting the germination of suitable pollen grains. We have studied the structure of the different component tissues of the pistil, their organization and cytochemical features of olive flowers, Olea europaea L., at anthesis, an essential first step for understanding the processes of pollen-pistil interaction and fertilization. The pistil from olive cv. Picual trees is characterized by a wet bilobed stigma, a solid style and a bilocular ovary containing four ovules. The stigma is composed of external multicellular papillae and a non-papillate inner region of secretory cells. An exudate is observed on the surface of the papillae at anthesis, the moment when the flowers (first) open, but the anthers are not yet dehiscent. The inner secretory cells of the stigma and those of the stylar transmitting tissue are continuous, constituting a funnel-shaped zone which extends from within the stigma to the style base. The outer surface of the ovary and style epidermis is surrounded by a cuticle layer, while internally, the locule wall, formed by the innermost cells of the endocarp, consists of two layers of periclinally oriented cells with thicker cell walls. Starch granules are distributed differentially, concentrated most densely in the style (adjacent to the vascular bundles), in the distil region of the ovary, and in the micropylar ends of the ovules. Well-developed vascular bundles are present in the lower part of the stigma, the style and in the pericarp of the ovary. The histochemical identification of sugars and lipid substances within and around the vascular bundles suggests that they are involved in the transport of these materials. Ultrastructural observations confirm the presence of exudates on the papillar surface and confirm the secretory characteristics of the inner stigmatic cells. They also demonstrate marked differences in size, form, and vacuolar and cytoplasmic contents among the cells of the various style and upper ovary tissues. We provide the first detailed cytological description at anthesis of all the olive pistil tissues, indicating the structural and cytochemical basis for the pistil behavior which will transpire during the progamic phase.